I can certainly see the wisdom in what they're telling you but it isn't necessarily correct. It can be done. How do I know? It happened to me.
A long story short, I bought a bike that was not quite right. Serial numbers filed off, nothing lining up correctly. It was only until we took the bottom end apart that it became apparent as to what was wrong. The cases were from two different engines. The crank was way off center. It was only after replacing the bearings and removing some of the dowels that hold the casings together did we get it to go back together within tolerance.
So it can be done but there's a lot of hassle and a bit of luck involved.
Rick.
